Another way to look at light guiding in a fibre is to measure the fibre's acceptance angle. This angle is the angle within which the light should enter the fibre optic to ensure it is guided through it. The acceptance angle is normally measured as a numerical aperture (NA).
The numerical aperture and acceptance angle measurements are a critical concern in practical fibre optics. Getting light into a fibre is known as "Coupling".
When fibre optics were first developed in the 1950s, no one believed that much light could be coupled into a single fibre. Instead they grouped fibres into bundles to collect a reasonable amount of light. Only when "LASERS" made highly directional beams possible did researchers seriously begin to consider using single optical fibres.
Figure shows light coupling into a fibre optic and the construction of a fibre optic cable.